The present invention relates to the treatment of oxidable gas generated from waste at a dumping area, in which the waste layer generating gas is covered with a sealing layer preventing the absorption of water and with at least one overlying material layer. Further, the invention relates to a dumping area structure, in which the said method is applicable.
In dumping areas for industrial and municipal waste, methane-containing gas is generated from organic waste in oxygen-free conditions. Gas flowing uncontrollably into the environment causes inconveniences in form of smell, problems in the landscaping of the dumping area, and in the worst case, explosion risks and fires as it accumulates to the structures. On a global level, methane releases into the atmosphere are problematic.
The bulk of biogas generated at dumping areas may be recovered to be utilised in the production of energy or as fuel for vehicles. The recovery requires a suction system, collection pipings, and a pumping plant for biogas. If the recovered gas is not utilised, it may be destroyed by torch burning. At small dumping areas, where gas does not cause immediate security risk or hazard to health, and where the generated amounts of gas are too small for the recovery to be economically profitable, the said active recovery systems will, however, become unreasonably expensive.
It is known that biological oxidation of gas occurs in the surface layer at a dumping area in favourable conditions, due to the effect of micro-organisms so that methane and oxygen react to form carbon dioxide and water; cf. for example Kightley D: Nedwell D. xe2x80x9cOptimising methane oxidation in landfill cover soilsxe2x80x9d. The Technical Aspents of controlled Waste Management, Department of the Environment, Report No CWM 114/94, Juli 1994, Humer M: Lechner Peter. xe2x80x9cGrundlage der biologischen Methanoxidationxe2x80x9d. WASTE REPORTS 05, Universitxc3xa4t fxc3xcr Bodenkultur Wien, and Maurice 1998, Landfill Gas Emission and Landfill Vegetation. Licenciate hesis. Lulea University of Technology. Department of Environment Engineering. Division of Landfill Science and Technology. 1998:01. In a similar way, gas may be oxidised in separate biofilters. Besides methane, micro-organisms may oxidise also stinking sulphur compounds.
According to a Finnish order of the authorities, which mainly follows the EU directive, after the filling area at the dumping area has reached its final height, a gas recovery layer, a sealing layer, a drying layer, and a surface layer have to be built on it in said order listed from down upwards. The purpose of the sealing layer is to prevent rainwater from becoming absorbed into the waste and from drifting to pollute water systems and groundwater. The purpose of the drying layer, which may comprise gravel or other similar coarse material permeable to water, is to collect the rainwater and direct it outside the dumping area. The sealing layer prevents the gas generated in the waste from erupting evenly through the surface layers of the dumping area so that also the biological oxidation of gas in the surface layer is prevented. If gas removal has not been arranged, gas may flow into the soil surrounding the dumping area, or it may break the sealing layer, which is why the demand for the controlled discharge of gases into the atmosphere is typically made on the filled-up dumping areas, if gas is not recovered for utilisation or burning. However, as methane is a strong greenhouse gas, it is hazardous to discharge it into the atmosphere.
The object of the invention is to provide a solution, with which methane and other oxidable gases generated at a dumping area may be treated to become harmless for the environment in cases in which the waste layer is covered with a sealing layer preventing the absorption of water and the discharge and/or diffusion of gas, and in which it would not be practical to build a separate gas recovery system for the utilisation of gas or for the destruction of it by burning. It is characteristic of the system according to the invention that gas is led in a self-operated manner through a penetration aperture formed to the sealing layer and that it is made to spread in the lateral direction to an upper material layer for the biological oxidation of gas by using one or more flow controllers arranged on and/or to the sides of the penetration aperture.
In other words, the object of the invention is to lead the gas, which naturally tends to rise upwards, in a controlled manner through the sealing layer and to spread it to the surface layer, in which micro-organisms oxidise the gas in a way known in it-self. With this solution, it is possible to reduce greenhouse gas releases, which are the result if gas is recovered in a known way to pipes or well structures from below the sealing layer and led directly into the atmosphere.
Thus, the invention is used for restricting greenhouse gas releases compared with the alternative that eases were recovered and led directly into the air; on the other hand. it is possible to achieve a considerable saving in costs compared with the alternative that gas were recovered from below the sealing layer to be destroyed by burning. Above all. these advantages concern small dumping areas or dumping areas generating only slight amounts of gas, with which the utilisation of gas to energy is unprofitable. The invention is an advantageous solution also in cases in which the methane content of gas is too low to be burned without supporting fuel. Because the invention is based on the self-operated transfer of gas from the waste to the oxidising surface layer, the advantage in investment and operating costs is considerable also in comparison with a conventional biological filter operated by means of pumping. Further, the invention may utilise the large oxidising capacity of the surface layer at the dumping areas, when again the capacity of a biofilter is restricted by its small-sized reaction vessel or other similar reaction space.
When required drying and surface layers are built on top of the sealing layer at a dumping area, the drying layer may preferably be utilised in spreading the gas to the surface layer, in which the oxidation occurs. The material of the drying layer, such as gravel, may in itself easily conduct gas, and it is possible to embed distribution pipes into the layer for intensifying the spreading of gas.
One solution for spreading the gas is to embed a plate-like piece into the upper layers above the penetration aperture made to the sealing layer at the dumping area. Such a plate may be placed horizontally, for example, between the drying layer and the oxidising surface layer. Alternatively, perforated pipes may be installed to extend radially to different directions from the penetration aperture to spread the gas in the lateral direction. The penetration aperture may comprise a sleeve-like piece penetrating the sealing layer, which may have a hollow interior, or which may be filled with material permeable to gas. A preferable solution is a well penetrating the sealing layer in the vertical direction, which is covered and from which perforated pipes spreading the gas extend radially to different directions. In these structures it has to be seen to that the edge of the well or sleeve extends above the sealing layer so that water is prevented from flowing to the waste layer under the sealing layer.
The invention may advantageously be applied by leading air and/or moisture into the oxidising surface layer at the dumping area, with which the bacterial action and the oxidation of gas are intensified.